1. Field of the Invention
The present invention relates to perovskite-type rare earth complex oxide combustion catalysts, a process for their preparation and their use in the fluid catalytic cracking (FCC) process of petroleum refining. More specifically, the present invention relates to perovskite-type rare earth complex oxide combustion catalysts with a support in which mullite (3Al.sub.2 O.sub.3. 2SiO.sub.2) is the main phase, a process for their preparation and their use as a catalyst for the combustion of carbon monoxide in the fluid catalytic cracking process of petroleum refining.
2. The Prior Art
Up to now the active components of a catalyst for the combustion of carbon monoxide in the fluid catalytic cracking process of petroleum refining have all been noble metals, such as platinum, palladium etc. Since the seventies the perovskite-type rare earth complex oxides used as the active components of the catalyst have attracted wide attention, and their catalytic oxidizing function for carbon monoxide and hydrocarbons have been confirmed (T. Nakamura, M. Misono, et al, Nippon Kagaku Kaishi 1679, 1980; N, Yamazoe, et al, Shokubai Vol. 25, 196, 1983). At present, catalysts using perovskite-type complex oxides as the active components are generally carried on .gamma.-Al.sub.2 O.sub.3 (U.S. Pat. No. 3,897,367, 1975; M. Ohasi, Shokubai Vol. 29, 598, 1989; Japan Patent Publications sho-53-7591; sho-61-274748) or carried on cordierite (N. Mizuno, et al, Chemistry Letters 1333-1336, 1986). However, the supports mentioned above will inevitably react chemically with the complex oxide; as a result, aluminum-containing perovskite or aluminum-containing spinel with non-catalytic activity is formed, causing part of the active components to be damaged, so that the activity of the catalysts dropped universally. In order to solve the problem, methods to increase the content of the active components or to deactivate the surface by pre-coating with active components have been adopted to maintain the desired activity. For example, N. Mizuno et al first impregnated La.sub.2 O.sub.3 onto cordierite, then carried the active components; some others use mullite after coating it with a layer of .gamma.-alumina or zirconia as the support (Japan Patent Publications, sho 60-51545). These methods make the manufacture of the catalysts complicated and increase their cost. Still others bypass this process by using the active components as catalysts alone without support. Obviously, such a method fails to give full play to the role of the active components. At present, in order to substitute the cheap rare earth complex oxides for noble metals (e.g. platinum, palladium etc.), people are trying to develop perovskite-type rare earth complex oxide combustion catalysts for treating automobile exhaust gas, so that the problem of noble metal shortage can be solved and production costs can be reduced. Nevertheless, no reports have appeared so far on the use of the above catalysts for the combustion of carbon monoxide in the catalytic cracking process of petroleum refining.